[WIP] Feature/use basis algorithms

This demonstrates how to use the tree-algorithms added in !55 (closed) to dune-functions and proposed in dune-typetree!2 (merged) by using it to implement bindTree().

Unfortunately it only works when making the non-const methods of BasisNodeMixIn public because you cannot list lambdas as friends. However, I believe that this is not a big problem, because we only ever handout a const tree such that these functions are hidden from the user anyway.

Caution: This is not meant to be merged!